1. Field of the Invention
This invention relates to an improved semiconductor manufacturing apparatus comprising a LP-CVD (Low Pressure CVD) device.
2. Description of the Related Art
FIGS. 1 and 2 of the accompanying drawings respectively schematically illustrate a horizontal type LP-CVD device and a vertical type LP-CVD device of the prior art. In FIG. 1, the horizontal type LP-CVD device comprises a heater 1, a chlorosilane gas nozzle 2, an ammonium gas nozzle 3, a reaction tube 4, an emission port 5 and wafers 6. On the other hand, the vertical type LP-CVD device shown in FIG. 2 comprises an outer tube 7, an inner tube 8, a furnace port flange 9, wafers 10, a dichlorosilane gas nozzle 12, an ammonium gas nozzle 13 and a heater 14.
When silicon nitride films are manufactured in an apparatus as shown in FIG. 1 or 2, the inside of the reaction tube 4 or the outer tube 7 is maintained at a low pressure between 20 and 50 (Pa) and dichlorosilane gas and ammonium gas are respectively supplied through the gas nozzles 2 and 3 in FIG. 1 or the nozzles 12 and 3 in FIG. 2 so that films are formed on the wafers 6 of FIG. 1 or the wafers 10 of FIG. 2 in the furnace (having the reaction tube and the heater) which has a temperature gradient between 770.degree. C. and 790.degree. C. realized by the heater 1 of FIG. 1 or the heater 14 of FIG. 2. It should be noted that the furnace port 4.sub.1 and the pump 42 (or the emission port 5) respectively constitute the lower and higher ends of the temperature gradient in FIG. 1, while the furnace port 7.sub.1 and the furnace bottom 7.sub.2 respectively constitute the lower and the higher ends of the temperature gradient in FIG. 2.
In FIG. 1, the reaction tube 4 is surrounded by the heater 1. The dichlorosilane gas nozzle 2 and the ammonium gas nozzle 3 respectively supply dichlorosilane gas and ammonium gas so that films are formed on the wafers 6 respectively. For formation of films, the gaseous reaction product is emitted from the emission port 5. Similarly in FIG. 2, the outer tube 7 is surrounded by the heater 14. Dichlorosilane gas and ammonium gas are respectively supplied from the dichlorosilane gas nozzle 12 and the ammonium gas nozzle 13 to form films on the wafers 10. The gaseous reaction product is emitted from the emission port 11 located outside of the inner tube 8. Boat 15 is moved in or out by raising or lowering the furnace port flange 9 by an elevator.
In short, with a horizontal type LP-CVD device or a vertical type LF-CVD device of the prior art, silicon nitride films are formed in an environment where a temperature gradient is present as described above. More specifically, U.S. Pat. No. 4,279,947 issued to Goldman et al. discloses an apparatus wherein a plurality of substrates are arranged in a reaction tube and dichlorosilane gas and ammonium gas are flowed and brought to react on the substrates in a vacuum condition between approximately 300 millitorr and 20 Torr and at temperature of approximately 650.degree. to 800.degree. C., a temperature gradient of approximately 100.degree. C. being realized along the gas flow.
Such a temperature gradient is required in an apparatus of the prior art to ensure an even thickness of the silicon nitride film product. If the film deposition process is conducted in a furnace where no temperature gradient is present, or under a temperature flat condition, the films produced near the furnace port will have a thickness which is greater than that of the films produced near the furnace bottom so that films with different thicknesses will be obtained as final products. In short, the temperature gradient is designed to offset the variation of thickness.
However, the provision of a temperature gradient in the furnace has certain drawbacks. First the etching speed of the silicon nitride film is inevitably dependent on the growth temperature of the film. The higher the growth temperature, and therefore the growth speed of the film, the lower the etching speed becomes.
Second the (expansion or contraction) stress of the silicon nitride film is also dependent on the growth temperature, so that the higher the growth temperatures, the smaller the stress. Therefore, films formed in a furnace having a temperature gradient have qualities which vary depending on where the wafers are arranged on the same boat.